The DNA sequence of herpes simplex type 1 (HSV-1) is known (references 13,25) and is linear with a length of about 152k residues. It consists of two covalently linked segments, designated long (L) and short (S). Each segment contains a unique sequence flanked by a pair of inverted terminal repeat sequences. The long repeat (RL) and short repeat (RS) are distinct. The unique long (UL) region includes genes UL1 to UL56, and the Us region includes genes US1 to US12.
A relatively large number of patients with advanced cancers will develop metastatic lesions in the brain and spinal cord. This frequently results in severe and debilitating neurological complications including headache, paralysis, seizures, and impaired cognition. It has been estimated that 70,000 cancer deaths occur each year in the United States with metastatic lesions to the central nervous system (CNS). Radiation and steroids are presently the principle therapies used, however, they are only palliative, and frequently cause significant neuropsychological and endocrinological morbidity. Surgery is generally reserved for removal of solitary metastases, and is often not curative (1).
Viral therapy for the destruction of tumors is not a new concept. Effects in various experimental tumor systems have been demonstrated using parvovirus H-1, Newcastle disease virus, retroviral vectors containing drug susceptibility genes, and Herpes Simplex Type I virus (HSV-1) (2-7). The mechanisms by which viruses improve the outcome in experimental tumor systems are complex and poorly understood. Brain tumors represent a dividing cell population occurring within an essentially non-dividing cell population of support cells, and terminally differentiated neurons. Thus, in the context of brain tumor therapy, one rationale is to select a virus that replicates exclusively or preferentially in dividing cells. Such a virus may be capable of establishing a lytic infection exclusively in tumor cells within the CNS, ultimately destroying the tumors without infecting surrounding brain, and without deleterious effects to the host.
Pioneering experiments with HSV showed a dose dependent improvement in survival of nude mice bearing intracranial human gliomas following intratumoral therapy with mutant HSV-1 dlspTK (3). This virus has a deletion in the viral thymidine kinase (TK) gene, (8) and exhibits a relatively neuro-attenuated phenotype in mice (9). However, dlspTK infection of tumor bearing animals causes histologically evident encephalitis (3). The use of TK' mutantsof HSV-1 for viral therapy also has an inherent major disadvantage in that these viruses are resistant to the clinically effective anti-viral agents acyclovir and ganciclovir (10).
The terminal 1 kb of the long repeat region (RL) of the HSV-1 and HSV-2 genomes contain a gene (11-13), that confers neurovirulence. Deletion or mutation of this gene (γ34.5), results in variants that grow as well as wild type virus on dividing cells of many established cell lines, but show impaired replication on non-dividing cells (12-14). In mice, γ34.5 null mutants are incapable of replicating in the central nervous system, and do not cause encephalitis (12,15-16).
A mutant HSV-1 called R3616, containing a 1000 base pair (bp) deletion in γ34.5, with an LD50 (minimum dose of virus that kills 50% of infected animals) that is at least 3×103 fold greater than wild type F strain virus from which it was derived (12), has been shown to improve the outcome of nude mice bearing intracranial human gliomas (17). In the work presented here, we have utilized an HSV-1 strain 17 mutant virus called 1716, that has a 759 by deletion in γ34.5 (16).
The construction of mutant virus 1716 is described in published patent application WO92/13943 (PCT/GB92/00179) the contents of which are incorporated herein by reference. However, this patent publication is solely concerned with the use of mutant 1716 as a vaccine, either in itself or as a vector vaccine which includes a heterologous gene coding for an antigen.
Melanoma is a prevalent malignancy. Cerebral metastases occur in up to 75% of patients with metastatic disease, and are among the most common causes of death (18-22). Presently, the life span of patients with CNS melanoma is short, ranging from 2 to 7 months (23).
It is an object of the present invention to provide an improved HSV-based viral therapy of cancer tumors.